Increasing the transmission capacity in optical networks requires wide band, narrow spacing optical components such as arrayed waveguide gratings (AWGs), operating at high bit rates. This places a number of constraints on the optical characteristics of the devices, including low chromatic dispersion.
Chromatic dispersion of a material relates to the speed that energy at different wavelengths travels through a material. Controlling the dispersion of arrayed waveguide gratings will be essential in 40 Gigabit per second (Gbps) transmission systems and in spectrally efficient 10 Gbps systems (50 GHz spacing). Having the ability to design the dispersion characteristics in an AWG offers significant advantages.
AWGs typically include an input waveguide, a slab free-propagation region, an arrayed waveguide grating, a slab-focusing region and output waveguides. Conventional AWG designs have obtained a flattened AWG intensity passband by applying a mode structure (MMI, parabolic horn, etc . . . ) to the standard AWG design at the entrance of one slab and a standard taper on the other slab. Because the passband is the result of the convolution between the two modes, the spectrum gets flattened. However, this design method leads to high dispersion and dispersion slope in the passband due to the non-flat phase of the multi-mode structure.
A second approach (Okamoto and Yamada, Optics Letters, Vol. 20, No. 1, Jan. 1, 1995) modifies the arrayed waveguide gratings to generate a sinc-like distribution that leads to a dispersion-free flattened passband. This approach uses a standard input/output in the slabs but modifies the amplitude distribution by adding some loss on some waveguides (amplitude) and changing the length of some waveguides (phase).
A low loss, very wide and flat-top AWG is described by Dragone in U.S. Pat. No. 6,195,482 (2001). The AWG is based on amplitude and phase modifications of the arrayed waveguides to give the desired spectral characteristics, and MMI (Multi-Mode Interference) tapers at the input and/or output of the device for minimum penalty loss.
In the case of flat-top AWG's based on a parabolic horn design, as described in the Dragone '482 patent, the specification of which is being incorporated herewith by reference, the chromatic dispersion (CD) over the 3 dB passband suffers a penalty as compared to Gaussian devices. In addition, CD increases 4 times with each halving of the channel spacing, thus easily exceeding typical values in the case of flat-top AWG's with spacing 50 GHz and less. For such AWG's, the CD should be compensated or reduced.
Various approaches have been proposed to modify the optical path lengths of AWGs. These include grooves formed in the optical paths for thermal compensation, as proposed by Inoue et al, U.S. Pat. No. 6,304,687; a patch for compensating birefringence induced by compressive strain, as in U.S. Pat. No. 5,341,444 to Henry et al; or replacing part of the silica core of the AWG pathways with polymeric inserts, also for temperature compensation.
Chromatic dispersion is due to non-linear phase variation across the passband. It is possible to modify (compensate) the phase in some pathways, or each pathway of the waveguide array of an AWG. This approach results in a reduction of CD, but not its complete elimination.